The performance of many electronic devices is often limited by the performance of the clocks they use. For example, the receiver in a global positioning system (GPS) works by attempting to align an internally-generated pseudorandom signal to an identical signal sent by a satellite and measuring the phase difference between the two signals. The receiver calculates the time required by the satellite signal to reach the receiver and thereby gives the distance between the satellite and the receiver. In this system, accurate timing is needed not only for precise determination of distance, but also for fast acquisition of the satellite signal. Other examples in which timing determines the ultimate performance of the system range from parallel analog-to-digital converters to spread-spectrum communications. Furthermore, frequency references provide the basis for a large number of other applications, such as digital communication, synchronization of networks, and power distribution.
In applications such as those identified above, an atomic clock would greatly enhance the performance of the system. Like quartz oscillators and clocks, atomic clocks function by generating a very stable frequency from a reference. The main difference is that a quartz oscillator derives its frequency from a mechanically vibrating reference, which makes the frequency sensitive to long-term changes in mechanical dimensions and stress. An atomic clock on the other hand derives its frequency from the energy difference between atomic states, which is a constant of nature and is therefore predictable and stable. Unfortunately, the size, power consumption, and difficulty to integrate atomic clocks with existing electronic devices have been the prohibitive factors for the use of atomic clocks in these devices.